The present invention will be described in detail in association with a configurable medical monitoring system. It is to be understood, however, that the invention is not limited to any such system.
Modern medical monitors are of the configurable type, i.e. they comprise a host system and parameter modules. The host system usually includes a cabinet containing the host processor, the operating system, a power supply and so on; a display; a keyboard and a plug-in cabinet or rack for the insertion of parameter modules. Further components like a printer or a plotter may also be included. The parameter modules contain the front end electronics required to measure a specific medical parameter such as ECG (electrocardiogram), respiration, blood pressure, temperature, blood gases and so on. For example, if we consider the ECG, the associated parameter module contains the electronics required for lead selection, a preamplifier and, if data transmission to the host system is performed in digital format, an analog-to-digital conversion circuit. Each of the parameter modules contains the front end electronics for at least one medical parameter of interest; but it is also possible to combine frequently used parameters (such as ECG and respiration) in a single parameter module.
The parameter modules have a connector jack for the insertion of an electrode connector, e.g. a connector which is connected with the ECG electrodes via a cable. The parameter modules include means which permit their insertion into a cabinet or a rack of the host system. This provides full flexibility in choosing only those parameters for monitoring which are required for a specific patient. In other words, the system can be "adapted" to specific clinical requirements, e.g. in the operating room, the recovery room, the intensive care unit or for a special kind of disease.
To provide electrical isolation, the parameter modules are usually connected with the host system via optical couplers.
The parameter modules further need clock or timing signals for the purpose of data acquisition. In order to obtain meaningful results, data acquisition has to be performed in equidistant intervals. Therefore, each parameter module has to be supplied with timing signals at fixed intervals which are used as start signals of an analog-to-digital conversion cycle.
Unfortunately, the intervals between said timing signals are not identical for all parameters, i.e., different parameter modules may require different sampling periods. For example, the ECG has to be sampled every 2 ms (milliseconds) in order to obtain a smooth and meaningful ECG waveform, while temperature need only be sampled every 32 ms.
To meet these timing requirements it would be possible to provide each parameter module with an internal timer which would generate the timing signals necessary for the data acquisition of said module. This, however, has the disadvantage of requiring a plurality of timers, which increases the cost and size of the system. Further, communication between the parameter modules and the host system is rather difficult in this case as this communication can only be performed on an asynchronous basis. Such asynchronous communications may generate several problems. For example, if every parameter module which has converted a sample of data into digital format is programmed to send an inquiry or interrupt to the host system in order to transmit its data, it may occur that certain inquiries overlap in time. If we assume, however, that data transmission from the parameter modules to the host system is only performed after an inquiry from the host system, the number of data samples which a certain parameter module has converted into digital format since the last inquiry is not necessarily constant. Therefore, the length of the communication between the host system and a certain parameter module may vary which further increases the difficulties in such a so-called "handshake" based system. Also, in such a system the parameter modules each need their own memory space for buffering of converted data samples.
The difficulties arising from asynchronous communication can be overcome by using a synchronous data transmission format, i.e. communication in fixed time intervals or slices and with a predefined number of bytes to be transmitted. With synchronous communication, however, internal timers cannot be used; timing signals must be provided either by a central clock or the host system. For the transmission of such timing signals, additional wiring and at least one additional optical coupler for every parameter module has to be provided. Due to the high cost of optical couplers and additional wiring, this solution is not feasible. Further, to avoid transmitting separate timing signals to the parameter modules, which require timing signals of differing intervals, the timing signal transmitted from the host system must be further individually divided or modified by the parameter modules to generate their appropriate timing signals. A solution to overcome this disadvantage is to use the end of a synchronous communication as a timing signal, i.e. a "start of conversion" signal. This is only possible, however, if not more than one parameter module is addressed during a certain time slice of the synchronous transmission. If more than one parameter module is addressed in each time slice and if the modules are not identical (which is necessarily the case in a multiparameter system with different analog to digital conversion cycles) the periods of the timing signals are not equal due to the varying length of the transmission times of the various parameters.